The instant invention relates to bimetallic disc-actuated thermostatic switches and more particularly to a bimetallic disc assembly and to a disc retainer therefor which are adapted to be assembled in a thermostatic switch for minimizing the response time of the switch.
It has generally been found that it is important for certain devices, such as coffee makers, copy machines and facsimile machines to include some means of protection against damage caused by thermal overload conditions. For this reason, devices of this type generally include thermostatic switches which are operative for interrupting the main heat generating circuitry components thereof in response to thermal overload conditions. However, it has been found that since thermal overload conditions can often develop very rapidly, it is essential for the thermostatic switches which are utilized in devices of this type to be operative with minimal response times in order to avoid damage resulting from thermal overload conditions.
The devices disclosed in the Craig, III U.S. Pat. No. 4,754,252 and Uehara U.S. Pat. No. 4,794,364 which represent the closest prior art to the subject invention of which the applicant is aware generally address the problem of minimizing the response times of bimetallic disc-actuated thermostatic switches to enable them to be more effectively utilized as thermal overload protection devices for various machines and appliances. However, while the devices disclosed in these references have generally been found to be operative with reduced response times, it has nevertheless been found that they are not operative with sufficiently rapid response times to make them satisfactory for all applications.
It has generally been found that in order to minimize the response time of a bimetallic disc-actuated thermostatic switch the bimetallic disc of the switch must be sufficiently exposed to the surrounding environment to enable the temperature of the disc to rapidly follow the temperature of the surrounding air. The aforementioned U.S. Patents to Craig, III and Uehara have generally addressed this requirement by providing enlarged openings in the end walls of the disc retainer caps thereof. However, it has now been found that in order to further minimize the response time of a bimetallic disc-actuated switch it is also important to minimize the amount of heat which is lost from the bimetallic disc of the switch through heat transfer to the elements which support and/or retain the bimetallic disc in position. It has been still further found that in order to minimize heat loss from the bimetallic disc of a switch it is not only important to minimize the areas of contact between the bimetallic disc and the disc supporting and/or retaining structure of the switch but it is also important to minimize the cross sectional areas of the thermally conductive paths between the disc supporting and/or retaining structure and the remaining portions of the switch.
The instant invention provides an improved disc assembly and disc retainer for a bimetallic disc-actuated thermostatic switch. In this regard, the disc retainer is adapted to minimize heat loss therethrough by minimizing the areas of contact between the disc retainer and a disc supported thereon and by also minimizing the cross sectional areas of the thermally conductive paths through the disc retainer to the remaining components of a bimetallic disc-actuated thermostatic switch in which the retainer is installed. In particular, the disc retainer of the instant invention comprises a central hub portion, a plurality of fingers which radiate outwardly from the central hub portion, a disc retainer projection adjacent the outer end of each of the fingers and a disc support shoulder adjacent each of the disc retainer projections. The disc retainer is adapted for receiving and supporting a bimetallic disc thereon so that the disc is supported on the shoulders on the fingers and retained in position by the disc retainer projections. However, because the retainer includes a plurality of fingers having open notches therebetween for supporting a bimetallic disc, the total cross sectional area of the thermally conductive paths through the retainer is limited to the sum of the cross sectional areas of those paths which pass through the fingers. As a result, the overall thermal conductivity through the retainer is reduced. The disc retainer includes at least three fingers and it preferably includes six fingers and the disc retainer is preferably integrally molded from a plastic material. Further, the disc retainer preferably has an axial aperture therethrough for receiving a transfer pin of a thermostatic switch so that the transfer pin is in engagement with the bimetallic disc for moving a moveable contact of the switch between open and closed positions.
The bimetallic disc assembly of the instant invention comprises a disc retainer of the above described type comprising a central hub portion and a plurality of fingers which radiate outwardly from the central hub portion, a bimetallic disc which received on the shoulders on the fingers and a cap element which is received on the disc retainer. The cap element preferably includes an end wall having an enlarged central opening therethrough so that the main portions of one side of the bimetallic disc are exposed to the surrounding atmosphere. The cap element preferably further includes a skirt portion having a plurality of apertures therethrough which are preferably substantially uniformly spaced around the circumference of the skirt portion. The skirt portion is adapted to be secured to the body portion of a thermostatic switch for retaining the disc assembly thereon. The arms of the disc retainer preferably have top and bottom faces and the apertures in the skirt portion of the cap are preferably oriented so that they are disposed partially above and partially below the bottom faces of the arms on the retainer so that the apertures function to reduce the total cross sectional area of the thermally conductive paths between the cap and the body portion of a switch on which the disc assembly is installed.
It has been found that the disc retainer and disc assembly of the instant invention can be effectively utilized for optimizing the response time of a bimetallic disc-actuated thermostatic switch. In particular, it has been found that the disc retainer and the disc assembly effectively minimize the total cross sectional area of the thermally conductive paths available for transferring heat to the body portion of a thermostatic switch and that as a result, they cooperate for enabling the bimetallic disc of the switch to retain a substantially greater portion of the heat transferred thereto from the surrounding environment. As a result, the bimetallic disc is capable of reaching its actuation point substantially more rapidly when it is exposed to a thermal overload condition so that the actuation time of a bimetallic disc-actuated thermostatic switch incorporating the retainer and disc assembly of the instant invention is substantially reduced.
Accordingly, it is a primary object of the instant invention to provide a disc retainer for a bimetallic disc-actuated thermostatic switch which is operative for minimizing the response time of the switch.
Another object of the instant invention is to provide an improved disc assembly for a bimetallic disc-actuated thermostatic switch which is operative for reducing the actuation time of the switch.
Another object of the instant invention is to provide a disc retainer for a bimetallic disc-actuated thermostatic switch which is operative for minimizing the cross sectional area of the thermally conductive paths between a bimetallic disc supported thereon and the body portion of a thermostatic switch on which the disc retainer is installed.
Another object of the instant invention is to provide a disc assembly for a bimetallic disc-actuated thermostatic switch comprising a cap which is adapted to reduce the total cross sectional area of the thermally conductive paths between the cap and the body portion of a thermostatic switch on which the cap is assembled.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.